1. Field of the Invention
The present invention relates to a method of and an active network feedback filter for reducing feedback action on a timely flow of current, which is drawn by a load from a network via rectifier.
2. Description of the Prior Art
Many new electric and electronic apparatuses, in particular, power units, switching power components, control gears, frequency converters, electronic commutated drives and the like draw more or less noticeable non-sinusoidal current when corresponding counter-measures are not taken. The current drawn from a network has a more or less high upper harmonic component. According to existing standards and regulations, the electrical apparatuses of the above-describe types, among them also electrical power tools, should draw from a public power network only a certain proportion of the current with an upper harmonic component. The counter-measures can include active or passive filters which bring with them, in particular for middle and high power-operated a noticeable increase in costs and weight of apparatuses equipped with such filters. For hand-held electrical power tools, often, providing such filters is associated with increased space requirements and assembly complications.
There exist numerous methods for bring the upper harmonics below a predetermined rate. There further exists a rough difference between passive and active filters. Passive filters consist, as a rule, of inductive resistors and capacitors connected downstream of a.c. side or of a rectifier and arranged on a d.c. side For active filters, different topologies are known. They include, as a rule, at least one electronic power switch. A survey of known methods and circuits for reducing network residual ripples, both passive and active, is given in an article of Dr. Richard Redl, "Achieving Compliance with New Line-Harmonics Regulations," ELFI S. A., documents of the seminar of PCIM '96, May 20, 1996, Nurnberg.
A further development of the active methods is known as a Power Factor Corrector (PFC) solution. It consists, in principle, in providing in a current path of a load a boost-converter, with which the current drawn from a network is regulated so that it becomes sinusoidal. Such a power factor correction is described in detail in the article of Michael Herfurth, "Active Upper Harmonic Filtering for a Network Rectifier with a High Output Power," Siemens Components 24 (1986), booklet 1, p.p. 9-13. This solution is also described in European Patents EP O 669 703 A2 and EP O 675 588 A2.
According to the method described in M. Herfurth's article, the network current, which flows through the inductive resistor of the seriesly connected choke, is converted into a current with a desired sinusoidal shape and having a triangular or ramp-shaped residual ripple. This residual ripple has a high-frequency component, which in order to satisfy the requirements of the Radio Interference standard, again should be attenuated, as a rule, with a passive filter. Here, there exists an inverse connection between expenditures associated with a boost-converter and a radio interference filter. The higher is the switching or commutation frequency or the inductance of the booster-choke the smaller is the residual ripple of the network current and, thereby, the smaller are expenditures associated with a radio interference filter. Even if a compromise can be reached, still with all active solutions and associated with them expenditures, in particular, for hand-held tools, the weight and volume of the inductive components is a noticeable drawback.
In order to further reduce the current residual ripple, without increasing the size of the booster-choke, the commutation frequency can be increased. However, with this, switching losses in the power switching transistor are increased, as disclosed in the M. Herfurth's article. With contemporary solutions, a conventional switching frequency lies in a frequency range from about 40 kilocycles to about 100 kilocycles.
Accordingly, an object of the present invention is a method and a circuit which permit to further reduce undesirable feedbacks which influence a timely flow of a current drawn from a network during operation of a load, in particular, when the current flows via rectifier.